The invention relates to heat exchange processes, but more particularly, the invention relates to a liquid heat exchanger interface and a process for making the same.
In a liquid heat exchanger interface for boiling liquid such as refrigerants, it is desirable from a thermodynamic viewpoint to have vaporization of the liquid take place with very little, if any, super heating of the bulk liquid. Open cell porous coatings are used on heat exchanger elements to thermodynamically affect how the liquid is vaporized.
A porous boiling surface coating in operation provides a multitude of interconnected partially liquid filled open cells which act as nucleation sites for the growth of a plurality of vapor bubbles of a boiling liquid. If the cells are not interconnected, their operation as nuclei for bubble growth is critically dependent on retaining entrapped air or vapor within the cells to initiate vaporization. However, with interconnected cells, vapor formed in a cell may activate one or more porously connected adjacent cells so that the cells are supplied with preferably a liquid film. Heat is transferred from the cell walls to the thin liquid film causing vaporization. Vapor bubbles grow and emerge from the interconnected cells and break away from the surface of the coating and rise through the liquid. Adjacent liquid flows by capillary action into the interconnected cells coating their walls. A high boiling coefficient results because only a thin film of liquid is being vaporized within the cells as opposed to super heating a thick layer of liquid to effect vaporization.
A porous coating per se does not effect a heat exchanger interface capable of promoting nucleate boiling. The coating or surface must have other certain physical requirements. For example, the cells must have a size that is capillarily responsive to the liquid to be vaporized, and the cells must be interconnected so they can be recharged with liquid after a bubble emerges. Also, the cells must be open to permit egress of vaporized liquid. The coating must provide a good conductive heating path so that sufficient transfer of heat may be made from the cell walls to liquid therein.
For example, a porous aluminum coating may be made by flame spraying round aluminum particles on a substrate using standard flame spray techniques. As disclosed in Metal Spraying and Sprayed Metal, W. E. Ballard, 1948, page 207, FIG. 153, a porosity of 34.3 percent is achievable with sprayed powdered aluminum. However, the cells are generally of the closed type and are not interconnected. Such a surface coating can enhance heat transfer only by an established increase in surface area. The techniques do not define an open cell coating structure where nucleation may be generated and propagated with capillary pumping of the liquid and ejection of vapor.
A prior art surface coating having the capability of establishing nucleation sites is disclosed in Conception of Nucleate Boiling with Liquid Nitrogen, Almgren and Smith (Paper from "Modern Developments in Heat Transfer", supplemental notes special summer program, Rohsenow and Bergles, MIT, 1968). As disclosed therein a heat transfer interface is prepared by sandblasting copper with a coarse abrasive so as to improve the mechanical bonding of flame-sprayed particles to the copper. Zinc and copper are simultaneously applied from two separate guns. The surface is etched in hydrochloric acid to remove the zinc and leave a porous, metallic surface layer of copper. Preparation of the surface requires extra steps of spraying from an additional gun and removing a sacrificial element, zinc. Structurally, the heat transfer path at the substrate copper interface is drastically reduced because particles of zinc are etched from the substrate. Also, if the zinc is not completely etched away, it may act as a contaminant to some working fluids.
U.S. Pat. No. 3,384,154 to Milton teaches a method of thermally bonding a porous layer or coating to a heat exchanger apparatus as an effective means for establishing a plurality of nucleation sites capable of promoting and sustaining nucleate boiling with very little super heat required. Although the coating as taught by Milton is quite good from the viewpoint of being capable of initiating and sustaining nucleate boiling, there are several problems or disadvantages associated with the thermal bonding by brazing, soldering, or sintering as taught in the specification and claimed. The thermal bonding of Milton requires the use of a third element which is either retained in the thermal bonding process (i.e. soldering or brazing) or sacrificed, (i.e. temporary binder or slurry). Another, but less preferred embodiment is a coating directly generated by sintering copper. The same type process would not work for the oxide film forming metals such as aluminum. The types of thermal bonding as taught by Milton are not readily applicable for economic manufacture using oxide film forming metals such as aluminum.
Soldering and brazing are akin to each other in that they both involve uniting separate metallic parts with a meltable alloy. Milton does not teach how particles can be brazed or soldered together to effect a porous coating or how the coating could be brazed or soldered to a heat exchanger surface. It can only be assumed that standard soldering and brazing techniques are used to thermally bond individual particles of the coating together and the coating or layer to the metallic surface of a heat exchanger. In either case, however, a third alloying element is involved which requires additional process steps to generate the surface. Moreover, many metals, such as aluminum, are very difficult to solder or braze especially in the size range of 40 to 400 mesh granular.
The sintering method used by Milton to thermally bond powdered metals together in such a manner to define a porous layer of coating requires the use of a sacrificial material such as isobutylene or methyl cellulose polymers. The temporary binders are mixed with the powdered material to form slurries which are used to facilitate distribution and hold the powder in place until a thermal bond is achieved and the binder is driven off. When the binder is driven off, the powders are simultaneously sintered.
It should be noted that some metal powders cannot be sintered unless special precautions are taken. These usually are the oxidized film forming metals such as aluminum. Special care must be taken to prepare such powders with additives that promote sintering or providing a reducing or inert atmosphere. In either case, a third element is involved in forming the coating which also requires additional process steps. Some metal powders such as copper may be sintered without the aid of a temporary binder. However, problems are involved in positioning and holding the powders in position for sintering and the interstices between particles are less controllable because pressure must be applied in such a sintering process. Moreover, sintering rounds and necks the interfaces between adjacent particles eliminating sharp crevices that would otherwise aid in the capillarity of the coating. Oxide film forming metal powder cannot be sintered without special process treatment. Aluminum is often sintered in an inert atmosphere or reducing atmosphere which requires special treatment or otherwise, additional process steps. When aluminum is sintered, the particles are compacted tightly against one another. Compacting precludes forming of an open celled interconnected structure which promotes nucleate boiling. Sintering aluminum particles having an aluminum oxide skin is also complicated by the fact that the temperatures required to sinter the aluminum oxide skin are considerably higher than the melting point of aluminum particles.